A semiconductor device includes a substrate including a recess, the recess being positioned below an isolation region and having a side portion including a plurality of stepped portions, a plurality of gate electrodes spaced apart from each other on the substrate, and stacked in a direction perpendicular to an upper surface of the substrate, a channel structure passing between a first set of the plurality of gate electrodes, and the isolation region passing between a second set of the plurality of gate electrodes, the isolation region extending from the upper surface of the substrate and having an inclined lateral surface.